This invention relates to a zoom lens system that is suitable for use with a compact camera and which is subject to less constraints on back focus than zoom lens systems for use with single-lens reflex cameras. More particularly, this invention relates to a compact zoom lens system of a structurally simple two-group telephoto type (hereinafter referred to simply as a "two-group type") which yet has a wide coverage of angles, i.e., ca. 37 degrees as half view angle, at the short focus end and which is capable of zooming up to a ratio of ca. 2.5 in consideration of its nature as a two-group type system having a wide coverage of angles. This invention also relates to a method of focusing with such a zoom lens system.
Conventional zoom lens systems for use with compact cameras are classified as two types, (A) a two-group type and (B) a three- or four-group type. Compared with type (A), zoom lens systems of type (B) have the advantage of requiring a relatively small amount of lens movement but, on the other hand, they are not only large in size but also complex in construction. Because of these obvious differences from lens systems of a two-group type which are envisaged by the present invention, type (B) will not be described in detail hereinafter.
Compared to type (B), zoom lens systems of type (A) require a somewhat greater amount of lens movement but because of their simple lens configuration and mechanical structure, type (A) zoom lens systems have the advantage of ease in size reduction. Conventionally known zoom lens systems of a two-group type include version (A-1) that is described in Unexamined Published Japanese Patent Application Nos. Sho-56-128911, Sho-57-201213, Sho-60-48009, Sho-60-170816 and Sho-60-191216, version (A-2) that is described in Unexamined Published Japanese Patent Application Nos. Sho-62-90611, and Sho-64-57222, and version (A-3) that is described in Unexamined Published Japanese Patent Application Nos. Sho-62-113120 and Sho-62-264019.
In the zoom lens system of the present invention, a negative lens element is used as the first lens and Unexamined Published Japanese Patent Application No. Sho-63-276013 describes a similar telephoto lens system of a two-group type in which a negative lens element is used as the first lens of the first group.
A method of focusing with such a two-group type zoom lens system is described in Unexamined Published Japanese Patent Application No. Hei-1-189620.
Version (A-1) has a small back focus and requires a large rear lens diameter, so it has had the problem that the overall size of the camera incorporating said lens system cannot be reduced. A further problem with this lens system is that internal reflections between the film plane and the last lens surface and other unwanted phenomena are highly likely to occur.
With a view to solving these problems, the assignee has proposed improved versions of a two-group type the back focus of which is comparatively large in consideration of its use with compact cameras. Such improved versions are (A-2) which is of a five-group-six-element composition and which is capable of a zoom ratio of 1.5-1.6 and (A-3) which is of a six-group-seven-element composition or seven-group-eight-element composition and which is capable of a zoom ratio on the order of 1.7-2.5. These versions range from a six-element composition capable of a zoom ratio of ca. 1.5-1.6 to an eight-element composition capable of a zoom ration of at least 2. However, they provide half view angle of only about 30 degrees at the short focus end and are chiefly intended to photograph scenery; in other words, they are short of satisfying the need to take pictures through wide angles with a compact camera. Further, those proposals have been unable to satisfy the need for providing a zoom lens system for use with a compact camera that is even more compact and less expensive.
The zoom lens system described in Unexamined Published Japanese Patent Application No. Sho-63-276013 uses a negative lens element as the first lens as in the present invention. However, this requires the use of lenses having a certain refractive index profile which are difficult to manufacture at low cost by the state-of-the-art technology. In addition, the half view angle that can be attained at the short focus end is no wider than 30 degrees.